High molecular weight aromatic polycarbonates and blends thereof with linear polyesters are known, and these can include finely divided particulate fillers having a low aspect ratio, such as silica, and polymeric particles, see, for instance, commonly assigned U.S. patent applications Ser. Nos. 553,172, now abandoned, 553,173, now U.S. Pat. No. 4,536,542 and 553,175, filed Nov. 18, 1983, now abandoned, Richard B. Allen. It is also known that foaming agents may be added to thermoplastics to produce a foamable material, as disclosed in U.S. Pat. Nos. 3,781,233, 4,097,421, 4,097,425, 4,280,005, and 4,351,911. These thermoplastics include polyesters and acrylonitrile-butadiene-styrene copolymers.
A difficulty which has been encountered is that when a foaming agent is added directly to a polyester, highly irregular foaming occurs which results in an inferior product containing voids and sink marks. A way to overcome this problem as disclosed in U.S. Pat. Nos. 4,280,005 and 4,351,911 is to add the foaming agent to a polyester containing a filler such as a fibrous glass, minerals or mixtures thereof in an amount of at least 5 percent.
Another difficulty, this time associated with polycarbonates, is that when a foaming agent is added, the material will not form a foam when injected into a cavity. Instead, the polymer collects as solid masses in the mold. Glass fibers can be added and these will to some extend alleviate the problem by aiding in the formation of gas bubbles that ultimately form cells, thus causing foaming. An entirely uniform, void-free structure is not produced, however, and the impact strength, including low temperature impact strength, of the molded article is adversely affected, which restricts the usefulnes of the composition in the range of foamed articles that can be produced.
The discovery has previously been made that structural foams comprising aromatic polycarbonate resins and related resins can be nucleated without the presence of glass fibers to yield foams that are superior in impact strength. This discovery is described in U.S. application Ser. No. 598,801, filed Apr. 10, 1984, now U.S. Pat. No. 4,544,677 Richard B. Allen and Roger W Avakian. The nucleating agent is an inorganic non-fibrous, particulate material, e.g., crystalline silica, alone or together with a particulate organic polymer, e.g., acrylate copolymer or polytetrafluoroethylene.